The invention relates to a high-gradient magnetic separator for the selective separation of magnetic particles from a suspension.
The separation of ferro-, ferri- or para-magnetic particles from liquid or gaseous fluids by magnetic separators is a basic concept of chemical engineering used in numerous variants. A particular advantage of the principle of the magnetic separation resides in the possibility to selectively separate magnetic particles from a mixture with other, non-magnetic particles. The selection of the magnetic separator is based on the size and the magnetic properties of the particles.
Relatively large and highly magnetic particles such as magnetite ores with particle sizes >75 μm can be separated with simple drum or band separators. Finer, strongly magnetic particles up to a size of 10-20 μm can still be separated from an aqueous suspension by special drum separators. Yet finer particles in the micrometer range (about 0.1 to 20 μm) have been separated so far only by so-called high-gradient magnetic separation procedures.
The principle of high-gradient magnetic separation is based on the generation and the bundling of high magnetic field gradients by the introduction of a ferro-magnetic matrix in an outer magnetic field. The magnetic elements of the matrix consist generally of steel wool or respectively a wire mesh or profiled metal plates. They are magnetized by the outer field and develop magnetic poles which at certain locations strengthen or weaken the outer magnetic field. The high field strength gradients formed thereby provide for a strong magnetic force effective on para- or respectively, ferro-magnetic particles directed toward higher field strengths. The particles attach themselves to the induced magnetic poles of the matrix and are thereby removed from the suspension.
With the generation of very high field gradients and correspondingly high magnetic forces in connection with a fine-mesh matrix, the method of high gradient magnetic separation is very effective if the amount of magnetic contamination to be removed from a suspension is small. Typically the method is used in the processing of kaolinites or in the removal of corrosion products from condensate circuits.
After a certain period of operation however, the separators are charged with separated magnetic particles to such a degree that the storage capacity of the magnetic separator is exhausted and the magnetic particles collected on the matrix have to be removed. The matrix is generally cleaned after the magnetic field has been switched off by a strong water jet or by back-flushing with high fluid flow speeds. Based on the form and design of the matrix which may consist for example of steel wool or layered wire webs or nets and which consequently has numerous interstices in the matrix area locally dead volumes are present which are not or only insufficiently flushed by the cleaning fluid. In addition, the desire to keep the volume of the flushing fluid as small as possible, and to hold the required pumping power down the amount of the flushing fluid used and the flow speed of the fluid that can be obtained during flushing are limited. As a result, removal of the particles is only incomplete. Particularly particles with a high remnant magnetism are hard to remove. Consequently, these particles continue to strongly adhere to the matrix wires, which detrimentally affects the clean-up efficiency to a significant degree.
While there is a multitude of patents and publications concerning the particle separation, only few examinations exist concerning the filter back-flushing and matrix cleaning. However, an effective and complete matrix cleaning is important and even essential for many applications if only to satisfy technical economical and ecological conditions. Particularly if the magnetic separation of magnetic particles is an important partial step of a continuous overall process, an optimal filter operation requires minimization of the matrix cleaning duration and of the flushing volume required herefor.
With certain applications, for example, in connection with water purification, a complete cleaning of the matrix is not absolutely necessary, although it is desirable and economically advantageous in order to fully utilize the separation capacity. The matrix is cleaned by high speed flushing water in a counter-current flow. U.S. Pat. No. 5,019,272 discloses a high gradient magnetic separator with a filter housing including a matrix which is rotated while the matrix is subjected to the flux of a permanent magnet. The filter matrix is cleaned by a combination of a pulsed-flow cleaning liquid, centrifugal faces and an alternating magnetic field. The rotational movement however, in this case, is not provided as an energy input means for the cleaning but for the generation of an alternating magnetic field on the basis of permanent magnets.
Based on this state of the art, it is the object of the present invention to provide a high-gradient magnetic separator which comprises a mechanically simple, sturdy, flexible and relatively inexpensive arrangement for an efficient cleaning of the matrix.